1. Field of the Invention
During the combustion of wood, coal, natural gas, mineral oil and other substances as well as during the smoking of cigarettes, non-combustible residues in the form of more or less fine suspended particles escape into atomphere. If the combustion process is not supplied with enough oxygen or if the combustion process is performed with a controlled deficiency of oxygen, as e.g. during the diffusion flame of a wood fire or during the glowing combustion of a cigarette, additionally coal (soot), carbonmonoxide and hydrocarbons are produced. In the hot part of the combustion, the core of the particles consisting of coal or ash is formed. As soon as the exhaust gases of the combustion cool down, the hydrocarbons, if present due to lack of oxygen, condense on the surface of these cores. Other, more volatile substances such as water or acids may condense together or on top of these. The chemical and toxical properties of the suspended particles, their ability to act as condensation nuclei and their role in the production of photochemical smog mainly depend on the nature of their surface, the thickness of the surface layer involved and on the nature of the process. In contrast, the diffusion of particles as well as their probability of deposition in a filter depends only on their size and not on their surface composition.
2. Prior Art
The quantitative characterization, i.e. the characterization of the size of aerosols, i.e. of particles which are suspended in a carrier gas, is well known in the prior art and does not offer any fundamental technical problems. Usually such measurements are performed by measuring the diffusion constant or the electrical mobility of charged particles. In contrast the chemical classification, i.e. the qualitative characterization of suspended particles, is very difficult to perform. Chemical characterization is possible with condensed particles only, whereby particularly the most important information about the nature of the substances present on the surface of the particles is lost. The mixture of unvolatile, high molecular mass hydrocarbons, e.g. polyaromatic hydrocarbons (henceforth in short called PAH), which is produced during combustion and which condenses on the surface of the particles, contains carcinogenic and other toxic compositions, which are only present in very small amounts, but which can create a very high activity, being exposed at the surface of the particles.
A surface sensitive method for the characterization of such particles as well as an apparatus for carrying out such method, therefore, is urgently needed, since such particles, mainly contained in exhaust gases of industry and of automobilies, represent the most important part of air pollution. Among hydrocarbons resulting from a combustion process there are always such substances which have, as surface layers or adsorbates, the tendency to easily supply electrons. This property results in enhanced chemical activity and in a low work function. In other words, they show a pronounced photoelectric effect.
The photoelectric effect of particles suspended in the atmosphere has been systematically examined, for the first time, in 1981 and has been described in the publication "H.Burtscher er al., J.Appl.Phys.53, 3787(1982)". The apparatus used there contains an ozone-free low pressure ultraviolet lamp. The air with the suspended particles flows past this lamp. Thereby, photoelectrically active particles emit electrons and the particles become positively charged. The charge on those particles create an electric current in a subsequent filter, which current is proportional to the photoelectric activity of the aerosol. In order to present condensation of water on the surface of insulation elements within the system, the incoming aerosol is heated to a temperature slightly above ambient.The presumption expressed in the aforementioned publication, namely that the particles of the atmosphere having a high photoelectric activity contain PAH, has been confirmed in subsequent examinations.
Further investigations have shown that other components found in the atmosphere like water, acids or aliphatic hydrocarbons may adsorb with, or on top of the PAH's and quench photoelectric activity. The apparatus described in the above publication, therefore, is not applicable to quantitative measurements because of the interference between those components activating photoemission with those quenching it.